Field of the Invention and Related Art Statement
The present invention relates to an apparatus for enhancing contours of a television signal, and more particularly to a contour enhancing apparatus which is advantageously installed in a television camera.
In a television camera a contour enhancer is generally provided for improving the sharpness of a reproduced image of an object, i.e. the definition of edges of the image to be picked-up. In the known contour enhancer, a brightness signal as shown in FIG. 1A is first differentiated to derive a contour signal as illustrated in FIG. 1B. In a color television camera, in general, the green color signal is used to derive the contour signal. Then, a contour enhancement signal is produced in proportion to the amplitude of the contour signal, and the contour enhancement signal is added to the original brightness signal to obtain a contour enhanced television signal shown in FIG. 1C. In case of the color television camera, the contour enhancement signal is added to the red, blue and green color signals, respectively. In the known contour enhancer, the amplitude or gain of the contour enhancement signal is proportional to that of the contour signal. Therefore, as illustrated in FIG. 1C, a contour enhancement signal having a small amplitude is added to a brightness signal having a small level difference and a contour enhancement signal of a large amplitude is added to a brightness signal having a large amplitude difference.
In the known contour enhancer, a video engineer can adjust the gain of the contour enhancement signal. For a general object, the brightness signal contains relatively small level differences, so that the gain of the contour enhancement signal is adjusted such that a contour having relatively small level differences in the brightness signal can be correctly enhanced so as to improve the sharpness of the image of the object. Therefore, when a brightness signal has a large level difference, a so-called over-enhancement might occur, and undesired glare might be manifest in an image reproduced on a monitor. Particularly, in a color television camera of the NTSC system, when the object contains frequency components near a subcarrier frequency of a composite color video signal, there might be produced large cross-color noise, and the quality of the reproduced image might deteriorate to a great extent. In order to suppress such cross-color noise, when the gain of the contour enhancement signal is decreased, the contour of a brightness signal having a small level difference could not be sufficiently enhanced, and the sharpness of the reproduced image might be lost.
The above mentioned cross-color noise is generated in the contour enhancement in the horizontal direction. It should be noted that when a brightness signal has a large level difference in the vertical direction, the vertical contour might be enhanced excessively. For instance, when cut surfaces of a diamond placed on a black velvet are to be picked-up, the contour of the diamond with respect to the black background is enhanced excessively, but the contrast of the cut surfaces might not be enhanced sufficiently. In this manner, it is difficult to obtain an image having the desired contrast. In such a case, when the gain of the contour enhancement signal is reduced so as not to enhance the contour of the diamond excessively, the contour of the cut surfaces might not be reproduced sufficiently.